Accompanying FIG. 1, which is an illustration of the known prior art, shows a conventional shaft-pinion assembly which is obtained by moulding a toothed wheel 5 made of injected plastics material, on a metal shaft 1 conventionally provided with longitudinal indents 2 for engagement.
The indents 2 are localized, on the periphery of the shaft 1, in the zone of reception of the moulded plastic toothed wheel 5.
At the other end of the shaft 1 there is provided, for example, in order to connect the article to be driven, such as a vehicle windscreen wiper for example, a knurled truncated cone 3 followed by a threaded cylinder 4.
A well known drawback of kinematic chains which use toothed wheels made of plastics material, such as the wheel 5 of FIG. 1, resides in the noise caused by such gears.
A particularly typical example resides in the devices for driving automobile windscreen wipers.
In the assembly of FIG. 1, it is question of a metal shaft 1 driven in rotation by the toothed wheel 5 made of plastics material.
The forces transmitted by the toothed wheel 5 to the shaft 1 may be considerable and the engagement 2 of this wheel 5 on the shaft 1 must therefore be extremely solid. The material of which this wheel 5 is constituted must consequently be a very hard plastics material, typically a glass-fiber reinforced material.
The toothed wheel 5 is typically driven by one or more other smaller pinions made of plastics material (not shown) which are themselves driven by a worm fixed on the shaft of the drive motor. It is generally at the gearing of these small pinions on the toothed wheel 5 that the noise is produced.
Document EP-A-0 303 113 may be cited as state of the art, which proposes to solve this difficulty by providing to use two different synthetic materials for making a toothed wheel similar to wheel 5 hereinabove:
a first thermoplastics material, made of hard matter, which ensures the desired rigid engagement on a metal shaft similar to shaft 1 hereinabove and which extends over a limited portion of the diameter of the wheel, PA1 a second thermoplastics material, made of less hard matter, but presenting vibration damping qualities, which surrounds the central halo formed by the first material and which forms the rest of the toothed wheel, outer toothing included. PA1 It is necessary to effect a solid engagement of the second portion of toothed wheel, or peripheral portion, on the first portion, or central portion. To that end, this rigid central portion must be provided with an outer engaging toothing on the peripheral portion, which renders the operation of manufacture delicate, all the more so as there may be problems of incompatibility between the hard material constituting this central portion and the less hard material constituting the peripheral portion. This increases the cost price of such a gearing. PA1 The outer drive toothing presents, ipso facto, limited mechanical strength qualities, with the result that such a toothed wheel wears out more rapidly than a wheel according to FIG. 1 entirely constituted by a hard thermoplastics material. PA1 characterized in that it consists in producing this gear by co-injection, through the same injection nozzle, of two thermoplastics materials of different mechanical characteristics:
However, this solution presents two drawbacks:
In the particular case of devices for driving the rear windscreen wipers of vehicles, it is also known to reduce the gearing noises by using a single toothed wheel of which the central portion is provided with cut-outs and openings. Such a solution is not valid for front windscreen wipers where the large toothed wheel is driven by one or more plastic reduction gears, which is not the case for rear windscreen wipers where the single toothed wheel is driven directly by the worm. In addition, the rear windscreen wipers are less stressed.
As state of the art concerning the reduction of noise in toothed wheels, document EP-A-0340 812 may also be cited, which describes a toothed wheel formed by two halves adapted to be assembled by screws along a plane of join which is orthogonal to the axis of the wheel. In order to dampen the vibrations, two conjugate recesses are provided so as to form, after assembly, an inner cavity, this cavity receiving two viscoelastic dampers with stress plate, well known per se, in particular in the domain of skis. It will be readily appreciated that, although this solution may prove satisfactory from the standpoint of results, it is not so from an industrial standpoint, due to its complexity and therefore its high manufacturing cost.
Finally, U.S. Pat. No. 3 630 098 may also be cited, which describes a rotating gear which comprises a polyamide core and an outer surface made of polyamide containing a lubricant.
This gear is manufactured by a process of injection in two successive, and clearly distinct, steps.